Separators play an important role in lithium-ion battery operation; however, no comprehensive studies of their microstructure and its impact exist. To enable such studies, we present a simple method for separator microstructure visualization and quantification based on focused-ion-beam scanning electron microscopic tomography. Here, we use this approach to visualize a sample of commercial polyethylene separator, calculate its directional effective transport parameters, and explain the impact of these values on battery performance. We further extend this technique to visualize metallic deposition within the separator, which could facilitate the study of lithium plating and dendritic growth. Separators in Li-ion batteries (LIBs) prevent electronic contact between positive and negative electrodes while allowing ionic transport between them.1 In the event of cell failure, separators limit ion transport to avoid internal short-circuiting and thermal runaway.2 To achieve good transport and suitable mechanical, thermal, and chemical properties, LIB separators are commonly <25 μm-thick sheets consisting of a complex 3D structure featuring 40% porosity and submicron pore sizes.2 Though they are routinely characterized by thickness, pore size, porosity, as well as thermal and electrical properties, 3 there has been no quantitative measurement of their 3D structure. Microstructural data would be particularly helpful in light of recent studies that link separator structure to the likelihood of LIB failure (e.g., through Li dendrite growth 4 ) and changes in separator structure (e.g., through pore closure by mechanical stress and chemical degradation 5,6 ) to aging effects in LIBs. Here, we present a simple procedure for visualizing and quantifying separator microstructures. We use simulations to assess the implications of separator microstructure on battery performance and extend our technique to examine lithium metal deposition within the separator.
ExperimentalWe employ focused ion beam scanning electron microscopic (FIB-SEM) tomography ( Figure S1 in the Supplementary Information 7 ), a technique that has been used for visualizing and quantifying of LIB electrode microstructures. 8,9 Figures 1a-1b show top view SEM images of polyethylene (PE) and polypropylene (PP) separators from Targray (PE16A and PP16). SEM images of cross-sectional cuts through the same separators (Figures 1c-1d) taken in the FIB-SEM configuration show pore edge contrast and depth information from the pores. Because these effects complicate image processing and quantification of the 3D microstructure, we in-fill the separator to enable imaging of a planar surface.Quantification of the 3D microstructure also requires sufficient contrast between the polymer skeleton and the in-filled pores. As a staining agent, we use OsO 4 , 10 which has previously been used with Celgard microporous membranes 11 as well as lithium dendrites and solid electrolyte interphases of graphite electrodes.12 PE and PP are inert toward OsO 4 staining, but OsO 4 reacts with unsa...